Photosensitive resin composition having a high refractive index

ABSTRACT

The present invention discloses a photosensitive resin composition which comprises (A) a compound having a molecule with at least one thiirane ring and a total number of a thiirane ring and/or an epoxy ring of at least 2 in the molecule, and (B) a photo acid generator, said composition having a refractive index of at least 1.6, and a method of obtaining a high refractive index periodical structure which comprises subjecting the photosensitive resin composition to photolithography.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photosensitive resin composition andan optical material using the same. More specifically, it also relatesto a refractive index periodical structure, and a process for preparingthe same, requiring a refractive index of at least 1.7, which is used inthe field of photonics, electromagnetic wave technology, micromachinetechnology, microreactor technology, and the like.

2. Description of Related Art

A refractive index periodical structure having a periodical distributionof a refractive index shows a diffraction/interference action to anelectromagnetic wave, and prohibits propagation of the electromagneticwave within a specific frequency region. This phenomenon corresponds toa band structure of electrons in a semiconductor crystal. Such arefractive index periodical structure is generally called a photoniccrystal. And, a frequency band region in which propagation is prohibitedis called a photonic band gap.

IT industries flourishing at the end of the 20^(th) century have beensupported by electronics technology based on semiconductor materialswhich control electrons, but are now faced with an essentially criticalsituation regarding technology. To further develop the technology in the21^(st) century, it is indispensable to transfer the technologicalviewpoint to a photonics technology to overcome the limitations of theelectronics technology.

Photonic crystals can control an electromagnetic wave; thereby obtaininga position to be a key material in the photonics technology similar tothe position of semiconductor material in the electronics technology.Photonic crystals are expected to be important elements for realizingoptical devices of the next generation such as ultra-high efficiencylasers, microminiature optical integrated circuits, and other usefuldevices.

In order for the photonic crystal to function effectively, it isnecessary to control the periodical structure with a space scale that isthe same as the wavelength of an electromagnetic wave, and obtain apredetermined value for a refractive index ratio of a high refractiveindex phase and a low refractive index phase. A minimum or lowestrefractive index ratio to be required may vary depending on the form ofthe periodical structure, and a larger value is generally preferred.

In the field of photonics, an object generally has a wavelength regionfrom a visible light region to a near infrared region. Therefore,photonic crystals are generally prepared having a period from asubmicron to a micron order. As a method to realize this, for example, amethod has been disclosed by Lin et al. for preparing woodpile (blocks)state photonic crystals in which square pieces made of Si have beenintegrated with an interval of several microns by making full use of thesemiconductor fine processing technology {Nature, Vol. 394, pp. 251-253(1998)}. Also, a wafer fusion technique has been disclosed by Noda etal. as a method for preparing woodpile state photonic crystals in whichsquare pieces made of GaAs or InP have been integrated with an intervalof several microns {App. Phys. Lett., Vol. 75, pp. 905-907 (1999)}.Moreover, Kawakami et al. succeeded in preparing submicron orderphotonic crystals having a specific three-dimensional periodicalstructure comprising Si and SiO₂ using an original bias sputterbuild-up/etching method called a self-cloning method {Electron. Lett.,Vol. 33, pp. 1260-1261 (1997)}. Furthermore, Vos et al. preparedsubmicron order inverse opal type photonic crystals by building uptitania in a space of an opal structure due to the self-assembling ofmonodispersed fine particles made of polystyrene according to thesol-gel method, and removing the polystyrene fine particles as molds bycalcination simultaneously with calcination of the titania {Science,Vol. 281, pp. 802-804 (1998)}. Also, Misawa et al. prepared submicronorder woodpile state photonic crystals comprising a photosetting resinby a two-photon absorption lithography process {Appl. Phys. Lett., Vol.74, pp. 786-788 (1999)}.

However, the above-identified methods each have their problems. Themethod of Lin et al. comprises many steps using complicatedsemiconductor fine processing techniques. The method requires the use oflarge sized apparatuses resulting in low productivity, high costs, andthe like, and not many types of materials can be applied to the method.Thus, it cannot be said to be a general method. The method of Noda etal. is an extremely excellent method since materials to be applied tothe method are abundant and flexibility and the structure is large.However, extremely severe conditions such as heating at 700° C. under ahydrogen atmosphere have been employed to carry out wafer fusion,resulting in safety concerns associated with this method. The method ofKawakami et al. is also an extremely excellent method since materials tobe applied to the method are abundant and productivity is high. However,it involves a serious problem that only a specific structure can beprepared and it cannot be applied to general use.

Opal type and inverse opal type photonic crystals can be extremelyeasily prepared, and have been widely used in research activities oflaboratories. However, these crystals have a low flexibility instructure, so that a certain breakthrough in a preparation method wouldbe indispensable to make a device. Also, from theoretical calculation,in an opal type and an inverse opal type photonic crystals, it isexpected that refractive index conditions required to form a completephotonic band gap are extremely severe compared to those required in thewoodpile state photonic crystals. Thus, they are disadvantageous fromthe point of flexibility in selecting materials. Also, in the inverseopal type photonic crystals, it is necessary to fill a material having ahigh refractive index into gaps of an opal mold.

However, this produces problems since it is difficult to fill thematerial into fine three-dimensional gaps uniformly and molds deformduring the accompanying filling.

As a method of preparing photonic crystals using a photocuring resin, ithas been proposed to use a usual optical modeling method in addition tothe method using the above-mentioned two-photon absorption lithography.In this method, a fine structure can be easily obtained, but therefractive index of the resin is low, at most 1.6 or so, so there is aproblem in that a large refractive index ratio cannot be obtained.

BRIEF SUMMARY OF THE INVENTION

The present invention is a photosensitive resin composition whichcomprises (A) a compound having a molecule with at least one thiiranering, and a total number of a thiirane ring and/or an epoxy ring of atleast 2 in the molecule, (B) a photo acid generator, and optionally (C)a plurality of fine particles having a refractive index of at least 2.0and an average particle size of 1 to 100 nm. The present invention alsoincludes the formation of a high refractive index periodical structureusing the photosensitive resin composition.

A refractive index periodical structure having a large difference inrefractive index is prepared by subjecting the photosensitive resincomposition having a refractive index of at least 1.6 to a rapidprototyping.

Optionally, a photosensitive resin composition having a refractive,index of at least 1.7 is formed by adding a plurality of fine particleshaving a refractive index of at least 2.0 to the photosensitive resincomposition of the present invention.

A high refractive index periodical structure is obtained by subjectingthe photosensitive resin composition having a refractive index of atleast 1.7 to photolithography.

BRIEF DESCRIPTION OF THE DRAWING

The features and advantages of the present invention will becomeapparent from the following detailed description of a preferredembodiment thereof, taken in conjunction with the accompanying drawing,in which:

FIG. 1 is an SEM image of a photonic crystal structure fabricated bytwo-photon lithography using a photosensitive resin compositionincluding (A) thiirane-modified BREN-S and (B) diphenyl iodoniumhexafluoroantimonate/3-(2′-benzimidazolyl)-7-N,N-diethyl-aminocoumarine.

DETAILED DESCRIPTION OF THE INVENTION

A photosensitive resin composition comprises (A) a compound having amolecule with at least one thiirane ring and a total number of athiirane ring and/or an epoxy ring of at least 2 in the molecule, and(B) a photo acid generator. Optionally, the photosensitive resincomposition may include (C) a plurality of fine particles, depending ona desired refractive index of the photosensitive resin composition.

A process for preparing the molecule with at least one thiirane ring isnot specifically limited so long as it can synthesize the compound. Apreferred preparation process may include a method in which an epoxyring of a corresponding epoxy compound to be used as a starting materialis converted into a thiirane ring by using a sulfide according tomethods known by a person skilled in the art. Here, “the correspondingepoxy compound” means a compound in which a sulfur atom of the thiiranering in an episulfide compound is substituted by an oxygen atom.Preferred sulfides may include a thiourea, a thiocyanate, and the like.Of these, thiourea and potassium thiocyanate are particularly preferred.The process should not be specifically limited to the number of epoxyrings converted into thiirane rings. More specifically, a methoddisclosed in, for example, J. M. Charlesworth, J. Polym. Sci. Polym.Phys., 17, 329 (1979) that uses a thiocyanate, or a method disclosed in,R. D. Schuetz et al, J. Org. Chem., 26, 3467 (1961) that uses thioureamay be used.

Examples of the epoxy compound having two or more epoxy groups in themolecule include, but are not limited to, a commercially availablebisphenol A type epoxy resin such as Epikote 1001, 1002, 1003, 1004,1007, 1009, 1010, 828 (all available from Japan Epoxy Resin Co., Ltd.);a commercially available bisphenol F type epoxy resin such as Epikote807(available from Japan Epoxy Resin Co., Ltd.); a commerciallyavailable phenol novolac type epoxy resin such as Epikote 152 and 154(both available from Japan Epoxy Resin Co., Ltd.), EPPN201 and 202 (bothavailable from NIPPON KAYAKU CO., LTD.); a commercially available cresolnovolac type epoxy resin such as EOCN-102, 103S, 104S, 1020, 1025, 1027,and Epikote 180S75(available from Japan Epoxy Resin Co., Ltd.); acommercially available brominated phenol novolac type epoxy resin suchas BREN-105, 304 and S (all available from NIPPON KAYAKU CO., LTD.); acommercially available cyclic aliphatic epoxy resin such as CY-75, 177,179 (all available from CIBA SPECIALITY CHEMICALS), ERL-4234, 4299,4221, 4206(available from U.C.C.), EPICLON 200, 400 (both available fromDAINIPPON INK AND CHEMICALS, INCORPORATED), Epikote 871, 872 (bothavailable from Japan Epoxy Resin Co., Ltd.), and the like. Of thesecompounds, the bisphenol A type epoxy resin, the bisphenol F type epoxyresin, the phenol novolac type epoxy resin, the cresol novolac typeepoxy resin or the aliphatic polyglycidyl ether are preferably used inview of the absence of coloring after heat treatment.

Most of the epoxy compounds mentioned above are high molecular weightcompounds, but the epoxy compound is not specifically limited by themolecular weight thereof, and, for example, a low molecular weightcompound such as glycidyl ether of bisphenol A or bisphenol F may beused.

A photo acid generator preferably in the form of an acid generation typecationic polymerization initiator, such as conventionally knownsulfonium salt, iodonium salt, phosphonium salt, diazonium salt,ammonium salt and ferrocene, are used. In the following, specificexamples are mentioned but the present invention is not limited by thesecompounds.

As a sulfonium salt series acid generation type cationic polymerizationinitiator, examples include, but are not limited to,bis[4-(diphenylsulfonio)phenyl]sulfide bishexafluorophosphate,bis[4-(diphenylsulfonio)phenyl]sulfide bishexafluoroantimonate,bis[4-(diphenylsulfonio)phenyl]sulfidebistetrafluoroborate,bis[4-(diphenylsulfonio)phenyl] sulfide tetrakis(pentafluorophenyl)borate, diphenyl-4-(phenylthio)phenylsulfonium hexafluorophosphate,diphenyl-4-(phenylthio) phenylsulfonium hexafluoroantimonate,diphenyl-4-(phenylthio)phenylsulfonium tetrafluoroborate,diphenyl-4-(phenylthio)phenylsulfoniumtetrakis(pentafluorophenyl)borate, triphenylsulfoniumhexafluorophosphate, triphenylsulfonium hexafluoroantimonate,triphenylsulfonium tetrafluoroborate, triphenylsulfoniumtetrakis(pentafluorophenyl)borate, bis[4-(di-(4-(2-hydroxyethoxy))phenylsulfonio)phenyl]sulfide bishexafluorophosphate,bis[4-(di-(4-(2-hydroxyethoxy)) phenylsulfonio)phenyl]sulfidebishexa-fluoroantimonate, bis[4-(di-(4-(2-hydroxyethoxy))phenylsulfonio)phenyl] sulfidebistetrafluoroborate, andbis[4-(di-(4-(2-hydroxyethoxy)) phenylsulfonio)phenyl]sulfidetetrakis(pentafluoro-phenyl)borate.

As the iodonium salt series acid generation type cationic polymerizationinitiator, examples include, but are not limited to, diphenyl iodoniumhexafluorophosphate, diphenyl iodonium hexafluoroantimonate, diphenyliodonium tetrafluoroborate, diphenyl iodoniumtetrakis(pentafluorophenyl)borate, bis(dodecylphenyl)iodoniumhexafluorophosphate, bis-(dodecylphenyl)iodonium hexafluoroantimonate,bis(dodecylphenyl)iodonium tetrafluoroborate, bis(dodecylphenyl)iodoniumtetrakis(pentafluorophenyl)borate,4-methylphenyl-4-(1-methyl-ethyl)phenyl iodonium hexafluorophosphate,4-methylphenyl-4-(1-methylethyl)phenyl iodonium hexafluoroantimonate,4-methylphenyl-4-(1-methylethyl) phenyl iodonium tetrafluoroborate, and4-methylphenyl-4-(1-methylethyl)phenyl iodoniumtetrakis(pentafluorophenyl)borate.

As the phosphonium salt series acid generation type cationicpolymerization initiator, examples include, but are not limited to,ethyltriphenylphosphonium tetrafluoroborate, ethyltriphenylphosphoniumhexafluorophosphate, ethyltriphenylphosphonium hexafluoro-antimonate,tetrabutylphosphonium tetrafluoroborate, tetrabutylphosphoniumhexafluorophosphate, and tetrabutylphosphonium hexafluoroantimonate.

As the diazonium salt series acid generation type cationicpolymerization initiator, examples include, but are not limited to,phenyldiazonium hexafluorophosphate, phenyldiazoniumhexafluoroantimonate, phenyldiazonium tetrafluoroborate, andphenyldiazonium tetrakis(pentafluorophenyl)borate.

As the ammonium salt series acid generation type cationic polymerizationinitiator, examples include, but are not limited to,1-benzyl-2-cyanopyridinium hexafluorophosphate,1-benzyl-2-cyanopyridinium hexafluoroantimonate,1-benzyl-2-cyanopyridinium tetrafluoroborate, 1-benzyl-2-cyanopyridiniumtetrakis(pentafluorophenyl)borate, 1-(naphthylmethyl)-2-cyanopyridiniumhexafluorophosphate, 1-(naphthylmethyl)-2-cyanopyridiniumhexafluoroantimonate, 1-(naphthylmethyl)-2-cyanopyridiniumtetrafluoroborate, and 1-(naphthylmethyl)-2-cyanopyridiniumtetrakis(pentafluoro-phenyl)borate.

As the ferrocene series acid generation type cationic polymerizationinitiator, examples include, but are not limited to,(2,4-cyclopentadien-1-yl)[(1-methylethyl)benzene]-Fe (II)hexafluorophosphate,(2,4-cyclopentadien-1-yl)[(1-methylethyl)benzene]-Fe (II)hexafluoroantimonate,2,4-cyclopentadien-1-yl)[(1-methylethyl)benzene]-Fe (II) tetrafluoroborate, and 2,4-cyclopentadien-1-yl)[(1-methylethyl)benzene]-Fe(II) tetrakis(pentafluorophenyl)borate.

Of these, the sulfonium salt and the iodonium salt series acidgeneration type cationic polymerization initiators are preferred fromthe viewpoints of curing rate, stability and economics. Incidentally,these photo acid generators may be used alone or in the co-presence of asensitizer.

Examples of commercially available sensitizers may include, for example,SP-150, SP-170, CP-66, CP-77 available from Asahi Denka Co., Ltd.;CYRACURE-UVI-6990, UVI-6974 both available from Union Carbide; CI-2855,CI-2639 both available from Nippon Soda Co., Ltd.; “Irgacure 261”(available from Ciba Specialty Chemicals Inc.(2,4-cyclopentadien-1-yl)[(1-methylethyl)benzene]-Fe (II)hexafluorophosphate); “RHODORSIL 2074”(4-methylphenyl-4-(1-methylethyl)phenyl iodoniumtetrakis(pentafluorophenyl)borate available from Rhone-Poulenc) and thelike.

As the plurality of fine particles having a high refractive index, ametal oxide powder or a metal salt powder are preferred, including, butnot limited to, titanium oxide (rutile), titanium oxide (anatase),potassium titanate, barium titanate, zirconium oxide, lead titanate,zinc sulfide, and zinc oxide is used. Of these, titanium oxide (TiO₂,refractive index; rutile: 2.76, anatase: 2.52), and zirconium oxide(ZrO₂, refractive index; 2.2) are preferred. The preferred plurality offine particles has a refractive index of at least 2.0. Titanium oxide isparticularly preferred from an industrial view point as well ascirculation and costs.

The particle size of the plurality of fine particles is preferably aslittle as possible from the view point of transparency of thephotosensitive resin composition which includes the fine particles.Preferably the photosensitive resin composition includes a plurality offine particles with a secondary average particle diameter of 200 nm orless, more preferably 100 nm or less, and an average primary particlediameter of the plurality of fine particles to be used as a startingmaterial is preferably 1 to 100 nm, more preferably 1 to 50 nm. If theparticle diameter is larger than 100 nm, the photosensitive resincomposition tends to be opaque to light having a wavelength more than400 nm because of Rayleigh scattering. The plurality of fine particleswhich have an average primary particle diameter less than 1 nm aredifficult to prepare due to technical limitation of nanopowderformation.

A surface treatment of the plurality of fine particles is preferredwhereby the preferred metal oxide powder is coated with a metallic oxidecompound, including, but not limited to, aluminum oxide, silicon oxide,and similar materials to provide dispersion stability in a polarsolvent.

It is also preferred to use a dispersant to stabilize dispersibility ofthe plurality of fine particles in addition to the surface treatment ofthe plurality of fine particles. The dispersant may include variouskinds of materials, including, but not limited to, those materialslisted below.

An aqueous dispersant may preferably include the following types ofmaterial. As an inorganic compound, a polyphosphate is preferably used.Among the polyphosphates, a condensed phosphate such as atripolyphosphate, a hexametaphosphate, a pyrophosphate, and similarpolyphosphates, or a silicate, for example, sodium silicate, is morepreferred. As a formalin condensate, a naphthalene sulfonate-formalincondensate, or a cresol sulfonate-formalin condensate is preferablyused. As a polymer, a polyacrylate, an olefin-maleate copolymerizedmaterial, or an acryl-maleate copolymerized material is preferably used,and the polyacrylate is more preferably used. As a naturalproduct-derived material, a lingo-sulfonate, a carboxymethylcellulosesalt, cationized starch, cationized cellulose, gelatin, dextrin, solublestarch, or skim milk is preferably used, and the carboxymethylcellulosesalt or gelatin is more preferably used.

As a nonionic dispersant, polyvinyl alcohol is a preferred dispersant.

As a non-aqueous dispersant, alkylbenzene sulfonate, sodiumdioctylsulfosuccinate, a partially amidated or partially esterifiedproduct of an olefin/maleic anhydride copolymer, an alkylimidazoline, analkyl amine acetate, an alkyldiamide, an alkylaliphatic acid salt or arhodinic acid salt is preferably used.

As a dispersing medium, various kinds of solvents including water, analcohol, a ketone, an ester, a halogenated hydrocarbon, an aromatichydrocarbon, an amide, and an ether may be used singly or incombination.

In a dispersing step, the plurality of fine particles, such as themetallic oxide powder, and the dispersant and the dispersing medium arecharged using a predetermined ratio. As a dispersing device, in additionto sand grinders, dyno-mills, ball mills, and the other various kinds ofmixers, continuous system or batch system mixing devices, millingdevices (i.e. mixing and kneading), and similar dispersing devices arepreferably used. When a kind or an amount of the dispersant is notlimited, the dispersing step can be carried out by using the usualvarious kinds of dispersing device. When an amount of the dispersant tobe added is restricted to a little and a conventionally used dispersingdevice for a low viscosity liquid such as a sand grinder, a dyno-mill,or a ball mill is used, there is a limit in a final particle size afterdispersion (i.e. an ultimate particle size), and this sometimes causesaggregation due to over-dispersion.

On the other hand, it is more preferred to prepare a fineparticles-dispersant material by a method in which a milled material isprepared by using a milling device which can disperse a material havinghigh viscosity with strong shear force and then diluting the material.It may be also possible to finely disperse a material by using adispersing machine for general use, for example, sand mill, aftermilling and dilution. With regard to a milling device that can bepreferably used in a system containing a little amount of a dispersant,there may be specifically mentioned, for example, a continuous kneader,an open kneader, a pressure kneader, a bunbury mixer, a three-roll mill,a planetary mixer, and the like. At the time of milling, by employing amethod in which a dispersing medium is first added in a slightlyexcessive amount to the above-mentioned composition, and after preparinga composition with thick concentration, an amount of the millingmaterial is decreased by utilizing spontaneous heat during the milling,under reduced pressure or vacuum or spontaneous evaporation, millingwith a strong load to the milling device can be carried out. It ispreferred to add a suitably amount of dispersing medium to the millingmaterial with a suitable interval to carry out milling under strong loadfor a long time.

When a large amount of dispersant is used, dispersibility of thedispersed material is improved but, in some cases, it exerts aninfluence upon various properties or a degree of milling.

When an amount of the dispersant is too little, grain stability aftermilling is poor and the dispersed material causes aggregation, so thatit is necessary to add an appropriate amount.

In the present invention, the preferred amount of dispersant mixed withthe plurality of fine particles, preferably a metallic oxide powder, is2 to 50% by weight, preferably 3 to 30% by weight, particularlypreferably 5 to 20% by weight.

A preferred range of the compound having a molecule with at least onethiirane ring to be used is not specifically limited, and it isgenerally 10 to 99.5 parts by weight, preferably 30 to 99 parts byweight based on 100 weight parts.

The photo acid generator, preferably an acid generation type cationicpolymerization initiator, can be selected from the above-mentionedmaterials. It may be used singly or in combination of two or more. Apreferred range of an amount of the photo acid generator, preferably theacid generation type cationic polymerization initiator, is notspecifically limited, and it is generally 0.05 to 25 parts by weight,preferably 1 to 10 parts by weight based on 100 parts by weight. If theamount to be added is less than 0.05 parts by weight, sensitivity isinsufficient, so that a remarkably large photo-irradiation energy or ahigh temperature treatment for a long period of time is required. Also,if it is added in excess of 25 parts by weight, improvement insensitivity cannot be expected and economically undesirable. Moreover,uncured components remain in a coated film with a large amount, so thatthere is a possibility of lowering in physical properties of the curedproduct.

A preferred range of an amount of the plurality of fine particles havinga high refractive index is 5 to 70 parts by weight based on 100 weightparts. If it is less than 5 parts by weight, an effect of increasing therefractive index is little, while if it exceeds 70 parts by weight, thestructure tends to be brittle.

The photosensitive resin composition can be used by mixing with asolvent including, but not limited to, acetone, methyl ethyl ketone,methylene chloride, toluene, methanol, ethanol, propanol, butanol,methylene glycol, ethylene glycol, propylene glycol, ethylene glycolmonomethyl ether and the like, or a mixed solvent of the above solvents,and the solution can be coated. The photosensitive resin composition maybe diluted with an epoxy or thiirane compound having a low viscosity,and the mixture may be used as such without using any solvent.

The photosensitive resin composition after coating is preferablyirradiated by an active light preferably using a rapid prototypingequipment, developed by a developer to give a cubic structure. Whenoptical modeling is carried out without any solvent, a cubic structurecan be obtained by rinsing an uncured portion by ethanol and the like.

As the active light to be used at this time, for example, those that caneffectively irradiate ultraviolet rays, including, but not limited to,carbon arc lamp, ultra-high pressure mercury lamp, high-pressure mercurylamp, xenon lamp, and the like may be used. In addition, He-Cd laser,argon laser, or femto-second Ti-sapphire extreme pulse laser may beused. When the femto-second Ti-sapphire extreme pulse laser is used, asubmicron order structure can be easily obtained by curing with atwo-photon absorption reaction. At this time, an infrared region from700 to 1000 nm can be used as an exposure wavelength. There is merit toeasily transmitting light even when it is a fine particles-dispersedsystem.

As the developer, those that are safe and stable, and have goodoperatability, such as a solvent that can dissolve the uncured portion,can be used. As the developing method, there are a dipping system, aspray system, and the like, which can be preferably used.

The photosensitive resin composition can be used, for example, as anoptical lens, a micro-lens array, a reflection preventive film, andother optical materials

EXAMPLES

Next, the present invention is explained by referring to Examples, butthe present invention is not limited by these Examples.

Synthesis of a compound having a molecule with at least one thiiranering is conducted as follows: Potassium thiocyanate (KSCN) (12.6 g) and10 ml of water were added to 15.0 g of BREN-S (NIPPON KAYAKU CO., LTD.)and dissolved in 100 ml of 1,4-dioxane. The mixture was stirred for 24 hat room temperature. A new portion of KSCN (12.6 g) and 10 ml of waterwere added to the reaction mixture and stirred for 24 h at roomtemperature. The mixture was extracted with ethyl acetate, washed withwater, and dried with MgSO₄. The solution was concentrated in vacuo andadded to isopropanol in order to precipitate the product. The productwas purified by reprecipitation from tetrahydrofuran (THF)/methanol. Theyield of a thiirane-modified BREN-S was 12.8 g (80%). The degree oftransformation to thiirane is 50% (±5) as calculated from ¹H-NMRspectrum. δ2.0-2.6 (m, —CH₂—S—, 6.0H), δ2.4-2.8 (m, —CH₂—O—, 6.0H),2.9-3.4 (m, —CH—S— and —CH—O— 6.0H), 3.5-4.2 (m, O—CH₂—C and Ar—CH₂-Ar,21.5H), 6.8-7.8 (m, Ar—H, 20H).

Example 1 to 3 and Comparative Example 1

Component (A)/Component (B): diphenyl iodoniumhexafluoroantimonate/3-(2′-benzimidazolyl)-7-N,N-diethylaminocoumarine(available from Aldrich Chemical Co.)/Component (D): dimethylformamidewere formulated to give a resin solution. In addition, a series of resinsolutions were prepared including Component (C): titanium oxide. Theweight ratio of the components in the series of resin solutions was50(A)/2(B)/1(C)/50(D). Formulations of the respective resin solutionsare shown in Table 1.

TABLE 1 Formulation Component A (formulated amount) Component (C)Example 1 Thiirane-modified BREN-S — Example 2 Thiirane-modified BREN-STitanium oxide (rutile) 5 vol % Example 3 Thiirane-modified BREN-STitanium oxide (rutile) 10 vol % Compar- BREN-S — ative example 1BREN-S: (brominated epoxy resin, available from Nippon Kayaku Co., Ltd.)

A refractive index of the photosensitive resin composition was measuredby spin coating the resin solution shown in Table 1 on a siliconsubstrate by using an ellipsometer (manufactured by Rudolph TechnologiesInc.). Measured wavelength was 830 nm.

Then, the resin solution was cast on a glass substrate and the solventwas removed by heating to 90° C. for 5 minutes. The resultingphotosensitive resin composition was subjected to scanning exposure byusing Ti-sapphire laser (Tsunami® manufactured by Spectra PhysicsLasers, U.S.A., irradiation wavelength: 750 nm, pulse width: 100femto-seccond or shorter). The sample after exposure was post-heated at110° C. for 5 minutes, and washed with tetrahydrofuran to give theobjective three-dimensional structure.

TABLE 2 Refractive index and resolution of the photosensitive resincomposition Refractive index (Measured wavelength: 830 nm) ResolutionExample 1 1.70 210 nm Example 2 1.75 210 nm Example 3 1.80 210 nmComparative 1.62 210 nm example 1

As shown in Example 1 of Table 2, a refractive index was raised bymodifying an epoxy resin to a compound having a molecule with at leastone thiirane ring. Also, by mixing titanium oxide fine particles withthe compound having a molecule with at least one thiirane ring, therefractive index could be further raised.

The photosensitive resin composition of the present invention has a highrefractive index, and is suitable as a photosensitive material for anoptical material.

In addition to the above effect, the photosensitive resin composition ofthe present invention can obtain a heightened refractive index by addinga plurality of fine particles having a high refractive index.

The photosensitive resin composition is a photosensitive material havinga high refractive index, and is suitable for obtaining a high refractiveindex periodical structure with photolithography. FIG. 1 is an SEM imageof a photonic crystal structure fabricated by two-photon lithographywith highly focused laser pulses at 750 nm. The photonic crystalstructure was formed using the photosensitive resin compositiondisclosed in Table 1, Example 1, which includes (A) thiirane-modifiedBREN-S and (B) diphenyl iodoniumhexafluoroantimonate/3-(2′-benzimidazolyl)-7-N,N-diethylaminocoumarine.Two-photon lithography easily produces a submicron fine structure.

Although the present invention has been disclosed in terms of apreferred embodiment, it will be understood that numerous additionalmodifications and variations could be made thereto without departingfrom the scope of the invention as defined by the following claims:

What is claimed is:
 1. A photosensitive resin composition comprising,(A) a compound having a molecule with at least two thiirane ring saidmolecule consisting of a thiirane-modified brominated phenol novolactype epoxy resin; (B) a photo acid generator, wherein said photo acidgenerator is selected from the group consisting of a sulflonium salt, aniodonuium salt, a phosphonium salt, a diazonium salt, an ammonium saltand a ferrocene; and (C) a plurality of fine particles having arefractive index of at least 2.0 and an average primary particlediameter of 1 to 100 nm.
 2. The photosensitive resin composition ofclaim 1, whereby said composition has a refractive index of at least1.6.
 3. The photosensitive resin composition of claim 1, wherein saidsulfonium salt is selected from the group consisting ofbis[4-(diphenylsulfonio)phenyl]sulfide bishexafluorophosphate,bis[4-(diphenylsulfonio)phenyl]sulfide bishexafluoroantimonate,bis[4-(diphenyl-sulfonio)phenyl]sulfidebistetrafluoroborate,bis[4-(diphenylsulfonio)phenyl]sulfide tetrakis (pentafluorophenyl)borate, diphenyl-4-(phenylthio)phenylsulfonium hexafluorophosphate,diphenyl-4-(phenylthio) phenylsulfonium hexafluoroantimonate,diphenyl-4-(phenylthio) phenylsulfonium tetrafluoroborate,diphenyl-4-(phenylthio)phenylsulfoniumtetrakis-(pentafluorophenyl)borate, triphenylsulfoniumhexafluorophosphate, triphenylsulfonium hexafluoroantimonate,triphenylsulfonium tetrafluoroborate, triphenylsulfonium tetrakis(pentafluorophenyl)borate,bis[4-(di-(4-(2-hydroxyethoxy))phenylsulfonio)phenyl]sulfidebishexafluorophosphate,bis[4-(di-(4-(2-hydroxyethoxy))phenylsulfonio)phenyl]sulfidebishexafluoroantimonate,bis[4-(di-(4-(2-hydroxyethoxy))phenylsulfonio)phenyl]sulfide-bistetrafluoroborate,and bis[4-(di-(4-(2-hydroxyethoxy))phenylsulfonio)phenyl]sulfidetetrakis -(pentafluorophenyl)borate.
 4. The photosensitive resincomposition of claim 1, wherein said iodonium salt is selected from thegroup consisting of diphenyl iodonium hexafluorophosphate, diphenyliodonium hexafluoroantimonate, diphenyl iodonium tetrafluoroborate,diphenyl iodonium tetrakis(pentafluorophenyl)borate,bis(dodecylphenyl)iodonium hexafluorophosphate,bis(dodecylphenyl)iodonium hexafluoroantimonate,bis(dodecylphenyl)iodonium tetra-fluoroborate,bis(dodecylphenyl)iodonium tetrakis(pentafluorophenyl)borate,4-methylphenyl-4-(1-methylethyl)phenyl iodonium hexafluorophosphate,4-methylphenyl-4-(1-methylethyl)phenyl iodonium hexafluoroantimonate,4-methylphenyl-4-(1-methylethyl)-phenyl iodonium tetrafluoroborate, and4-methylphenyl-4-(1-methylethyl)phenyl iodoniumtetrakis(pentafluorophenyl)borate.
 5. The photosensitive resincomposition of claim 1, wherein said phosphonium salt is selected fromthe group consisting of ethyltriphenylphosphonium tetrafluoroborate,ethyltriphenylphosphonium hexafluorophosphate, ethyltriphenylphosphoniumhexafluoroantimonate, tetrabutylphosphonium tetrafluoroborate,tetrabutylphosphonium hexafluorophosphate, and tetrabutylphosphoniumhexafluoroantimonate.
 6. The photosensitive resin composition of claim1, wherein said diazonium salt is selected from the group consisting ofphenyldiazonium hexafluorophosphate, phenyldiazoniumhexafluoroantimonate, phenyldiazonium tetrafluoroborate, andphenyldiazonium tetrakis(pentafluorophenyl)borate.
 7. The photosensitiveresin composition of claim 1, wherein said ammonium salt is selectedfrom the group consisting of 1-benzyl-2-cyanopyridiniumhexafluorophosphate, 1-benzyl-2-cyanopyridinium hexafluoroantimonate,1-benzyl-2-cyanopyridinium tetrafluoroborate, 1-benzyl-2-cyanopyridiniumtetrakis(pentafluorophenyl)borate, 1-(naphthylmethyl)-2-cyano-pyridiniumhexafluorophosphate, 1-(naphthylmethyl)-2-cyanopyridiniumhexafluoroantimonate, 1-(naphthylmethyl)-2-cyanopyridiniumtetrafluoroborate, and 1-(naphthylmethyl)-2-cyano-pyridiniumtetrakis(pentafluorophenyl)borate.
 8. The photosensitive resincomposition of claim 1, wherein said ferrocene is selected from thegroup consisting of (2,4-cyclopentadien-1-yl)[(1-methylethyl)benzene]-Fe(II) hexafluorophosphate, (2,4-cyclopentadien-1-yl)[(1-methylethyl)benzene]-Fe (II) hexafluoro-antimonate,2,4-cyclopentadien-1-yl )[(1-methylethyl)benzene]-Fe (II)tetrafluoroborate, and 2,4-cyclopentadien-1-yl)[(1-methylethyl)benzene]-Fe (II) tetrakis(pentafluorophenyl)borate. 9.The photosensitive resin composition of claim 1, wherein saidcomposition has a refractive index of at least 1.7.
 10. Thephotosensitive resin composition according to claim 9, wherein saidcomposition is cured by irradiation of active energy light.
 11. A methodof obtaining a high refractive index periodical structure comprisingsubjecting the photosensitive resin composition of claim 10 tophotolithography
 12. A method of obtaining a high refractive indexperiodical structure comprising subjecting the photosensitive resincomposition of claim 9 to photolithography.
 13. The photosensitive resincomposition of claim 1, wherein said plurality of fine particles has asurface coated with a metallic oxide compound.
 14. The photosensitiveresin composition of claim 13, wherein said metallic oxide compound isaluminum oxide or silicon oxide.
 15. The photosensitive resincomposition of claim 1, wherein said plurality of fine particles is ametal oxide powder or a metal salt powder.
 16. The photosensitive resincomposition of claim 15, wherein said plurality of fine particles isselected from the group consisting of titanium oxide, potassiumtitanate, barium titanate, zirconium oxide, lead titanate, zinc sulfide,and zinc oxide.
 17. The photosensitive resin composition of claim 1,further comprising a dispersant to stabilize dispersibility of saidplurality of fine particles.
 18. The photosensitive resin composition ofclaim 17, wherein said dispersant is selected from the group consistingof a tripolyphosphate, a hexametaphosphate, a pyrophosphate, anaphthalene sulfonate-formalin condensate, a cresol sulfonate-formalincondensate, a polyacrylate, an olefin-maleate copolymerized material, anacryl-maleate copolymerized material a lingosulfonate, acarboxymethylcellulose salt, cationized starch, cationized cellulose,gelatin, dextrin, soluble starch, skim milk, polyvinyl alcohol,alkylbenzene sulfonate, sodium dioctylsulfosuccinate, a partiallyamidated or partially esterified product of an olefin/maleic anhydridecopolymer, an alkylimidazoline, an alkyl amine acetate, an alkyldiamide,an alkylaliphatic acid salt and a rhodinic acid salt.
 19. Thephotosensitive resin composition of claim 17, further comprising adispersing medium for said plurality of fine particles.
 20. Thephotosensitive resin composition of claim 19, wherein said dispersingmedium is selected from the group consisting of water, an alcohol, aketone, an ester, a halogenated hydrocarbon, an aromatic hydrocarbon, anamide, and an ether.
 21. The photosensitive resin composition of claim1, wherein said compound is 10 to 99.5 parts by weight based on 100weight parts of the photosensitive resin composition.
 22. Thephotosensitive resin composition of claim 21, wherein said compound is30 to 99 parts by weight based on 100 weight parts of the photosensitiveresin composition.
 23. The photosensitive resin composition of claim 1,wherein said photoacid generator is 0.05 to 25 parts by weight based on100 weight parts of the photosensitive resin composition.
 24. Thephotosensitive resin composition of claim 23, wherein said photoacidgenerator is 1 to 10 parts by weight based on 100 weight parts of thephotosensitive resin composition.
 25. The photosensitive resincomposition of claim 1, wherein said plurality of fine particles is 5 to70 parts by weight based on 100 weight parts of the photosensitive resincomposition.
 26. The photosensitive resin composition of claim 1,further comprising a solvent.
 27. The photosensitive resin compositionof claim 26, wherein said solvent is selected from the group consistingof acetone, methyl ethyl ketone, methylene chloride, toluene, methanol,ethanol, propanol, butanol, methylene glycol, ethylene glycol, propyleneglycol, and ethylene glycol monomethyl ether.
 28. The photosensitiveresin composition according to claim 1, wherein said composition iscured by irradiation of an active energy light.
 29. A method ofobtaining a high refractive index periodical structure comprisingsubjecting the photosensitive resin composition of claim 28 tophotolithography.
 30. A method of obtaining a high refractive indexperiodical structure comprising, subjecting said photosensitive resincomposition of claim 1 to photolithography.